President Obama is caught between two important constituencies as he recalibrates his policy in Afghanistan -- the generals who want more troops, and the base of his own party, whose tolerance for a worsening conflict is quickly evaporating.

As the Obama administration prepares for a report from its senior field commander that is likely to request additional forces, congressional Democrats, in particular, have begun to question the wisdom of further reinforcements on top of the 62,000 U.S. troops already deployed in Afghanistan, with an additional 6,000 scheduled to arrive by year's end. The criticism comes as international fatalities in Afghanistan have risen to historic highs after a presidential election undermined by Taliban violence and low voter turnout.

The U.S.-led coalition in Afghanistan marked a grim milestone Tuesday when four American troops died in a roadside explosion near the volatile southern city of Kandahar. The attack brought the number of foreign troops killed in Afghanistan this year to 295, making this, with more than four months to go, the deadliest year for international forces since the war began in 2001. Americans account for 172 of the deaths this year, compared with 155 U.S. troop deaths in all of last year.

The domestic criticism is largely coming from those in the left wing of Obama's party, who say the president's plan to send more troops, monetary assistance and civilian advisers to Afghanistan does not include a well-defined exit strategy. Sen. Russell Feingold (D-Wis.), a member of the Senate Foreign Relations Committee, called this week for the first time for Obama to set a "flexible timetable" to withdraw U.S. forces, saying he is "not convinced that simply pouring more and more troops into Afghanistan is a well-thought-out strategy."

During last year's campaign, Obama made clear that he intended to draw down U.S. forces in Iraq to focus resources on winning in Afghanistan, despite the Democratic base's consistent opposition to doing so. He removed Gen. David D. McKiernan as the top U.S. commander in Afghanistan in May and replaced him with Gen. Stanley A. McChrystal, who is expected to request as many as 25,000 additional troops in a report to the president due next week.

In a speech to veterans this month, Obama reiterated his view that the United States is fighting a "war of necessity" in Afghanistan. He said his strategy was to deny al-Qaeda and its affiliates safe haven in the region, protect nuclear-armed Pakistan from a Taliban insurgency and bring a measure of stability to Afghanistan. A number of congressional delegations visited the country during their legislative recess this month and are reporting back their findings.

Congressional Democrats' calls for a strategic rethinking have coincided with a downward turn in U.S. public opinion toward the war, which will mark its eighth year in October. A Washington Post-ABC News poll published last week showed that a majority of Americans do not think the war is worth fighting and that nearly one-third think the United States is "losing."

"Afghanistan is going to be a huge political challenge. There's no doubt about that," said a senior administration official, who spoke on the condition of anonymity because the military assessment is pending. "The key for us is to have a strategy and have the competency to execute that strategy. It's going to be hard to convey this. And it's never going to be popular."

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, reiterated his concern Tuesday about the decline in U.S. public support for the Afghan war, which he called vital earlier in the week. But he invited a national debate over the conflict, saying it was better to take a "hard look" at the problem than to ignore it.

"I've seen the public opinion polls saying that a majority of Americans don't support the effort at all," Mullen told an audience of thousands of veterans at the American Legion convention in Louisville. "I say, good. Let's have that debate, let's have that discussion."

"Let's take a good, hard look at this fight we're in, what we're doing and why," he said. "I'd rather see us, as a nation, argue about the war -- struggling to get it right -- than ignore it."

Mullen said in an interview that with the "right resources," the coalition could begin to make progress in quelling the insurgency within 12 to 18 months.